The crystallinity, crystallite size and initial modulus of high-tenacity nylon 6 filaments annealed with constraint at various temperature (120。∼200$^{circ}C$) and time (6 sec ∼1800 sec) were measured by density, wide angle X-ray diffraction method and stress-strain curves. The results are as follows. 1) The crystallite size on the 004 plane (fiber axis) increases with increasing temperature and time. 2) The crystallinity increases with increasing temperature and time, and it may be attributed to growth of existing crystallite. 3) The initial moduli edcrease with increasing temperature and time, and more remarkable dependence on temperature than that of treating time is recognized.